Poly .alpha.-olefins (PAOs) are known to be useful as synthetic lubricants. PAO base stocks are typically prepared by polymerization of .alpha.-olefins, followed by hydrogenation to remove residual unsaturation. The polymerization of the .alpha.-olefins is usually performed in the presence of a Lewis Acid catalyst such as boron trifluoride or aluminum chloride, in complex with a promoter such as water, an alcohol, an ester, or an acid. See for example U.S. Pat. No. 3,742,082 to Brennan.
Typically, a polar ester base stock is added to PAO base stocks to improve additive or sludge solvency, volatility and/or seal swell characteristics. However, these polar esters suffer from the disadvantages of being hydrolyzable in the presence of water, and of having thermal and oxidative stabilities that are inferior to PAOs.
However, esters involve relatively expensive material costs and complicated production processes. A lower cost additive would be greatly desired.
Alkylbenzene base stocks are known in the art. See "Synthetic Lubricants and High-Performance Functional Fluids", Shubkin, R. L., Ed., Marcel Deker, Inc., New York 1993. However, alkylbenzene base stocks have not been used extensively in lubricant formulations because they are produced as an undesirable side-product in the production of detergent alkylates, and thus both their supply and their quality are unreliable.
Arylated PAO base stock synthetic lubricants are known to improve hydrolytic and thermal oxidative properties as compared to esters. However, until now, their preparation has been fraught with difficulties which resulted in low yields and poor, unreliable quality.
In the past, olefins and aromatic compounds were reacted in a single reaction step to provide an intermediate mono-alkylbenzene. See P. R. Pujado, Linear Alkylbenzene (LAB) Manufacture, Chapter 1.5.; Handbook of Petroleum Refining Processes, 2nd Ed., Editor--Robert A. Meyers, McGraw Hill, N.Y. 1977. While monoalkylates are useful for detergent alkylate, they are not suitable for lube stock because these detergent alkylates are usually made from benzene alkylation with C.sub.10 to C.sub.16 olefins, and their total molecular weight range only from 218 (C.sub.10 -B.sub.z) to 302 (C.sub.16 -B.sub.z). Some typical commercial LAB fluids have boiling ranges of from about 553 to about 580.degree. F., and their 100.degree. C. viscosities are usually less than 3.0 cS. These LAB are too volatile, and possess viscosities that are too low for high quality lube applications. Thus, in order to produce a suitable lube stock with very low volatility and a wide range of viscosities to provide adequate lubricity, a second alkylation step is required, which results in decreased yield.
Another advantage of this invention is that the fluid described here has properties superior to a simple PAO fluid. PAOs are currently produced from polymerization of .alpha.-olefins followed by hydrogenation to remove unsaturated double bonds, to produce a finished PAO base stock. The finished base stock is purely a paraffinic fluid, and therefore has very poor additive or sludge solvency and dispersancy. In most synthetic lubricant PAO formulations, an ester is used as co-basestock to improve the solvency and dispersancy of PAO base fluid. By adding an aromatic component to the PAO structure, one can significantly improve solvency and dispersancy, allowing the formulation of synthetic products without using an added ester, thus eliminating the problem associated with ester fluid, such as poor hydrolytic stability.
Accordingly, improved arylated poly .alpha.-olefins and methods for their preparation are needed. This invention is directed to this important end.